1. Field
The present description relates to semiconductor photolithography and, in particular, to generating alternating phase shift photolithography masks.
2. Background
In the production of semiconductors, such as memory, processors, and controllers, among others, a mask is used. The mask pattern is projected onto a semiconductor wafer to expose or shield different portions of the wafer from light, or some other element. The exposed wafer is then processed with etching, deposition and other processes to produce the features of the various semiconductors in the wafer that make up the finished product.
The masks are designed using computer design programs that derive an aerial view or image of the wafer based on the electronic circuitry that is to be built on the wafer. The mask is designed to produce this aerial image on the wafer in the particular photolithography equipment that is to be used. In other words the mask must be designed so that when illuminated with a particular wavelength of light at a particular distance is directed to a wafer through a particular set of optics and the mask, the desired pattern will be formed with the desired intensity on the wafer.
By making the features on a semiconductor smaller, more and faster processing power can be provided in a smaller space. At the same time the energy consumed and the heat produced by the chip is reduced. Smaller features require higher resolution from the mask and optical system that creates the pattern on the wafer. Resolution enhancement techniques (RET) are used to create smaller patterns.
Alternating phase shift masks (APSM) is one type of RET. It uses two independent exposures. For APSM, the design layout pattern is broken up into two independent layers. The first layer is converted to an APSM by an algorithm which assigns phases to create the desired pattern. In general, a complex pattern cannot be assigned phases without conflicts. A phase conflict can result in error in the printing of the features. These phase conflicts are corrected by a second exposure using a trim mask that removes most of the phase conflict errors but limits the resolution.
Optical proximity correction (OPC) is also used to enhance resolution and accuracy for small features. In OPC, the mask pattern is supplemented with very small subresolution features that are used to improve the accuracy with which the pattern is actually produced by the photoresist. These features may include pattern decorations and scatter bars. While the quality of the pattern and the performance of the resulting circuit is improved, the resolution is not significantly increased.